1. Technical Field of the Invention
This invention relates to a drill bit made of cemented carbide and more particularly to a drill having a construction which compensates for the brittleness of cemented carbide to increase the cross-break strength while decreasing the cutting resistance and simultaneously improving chip ejecting function or ability of the drill bit.
2. Description of the Prior Art
Generally, drills or drill bits of high speed steel have heretofore been used in drilling steel and cast iron work pieces. However, today when there is much need to maximize the efficiency of drilling operations, there are many cases where the drill rpm (cutting speed) is increased to meet this need. Under such circumstances, increasing use is being made, as a drill material, of cemented carbide, which is superior in wear resistance. However, cemented carbide has an inferior in cross-break strength compared to high speed steel and hence it is not a satisfactory material so far as the strength required for withstanding the cutting resistance is concerned. For this reason, with the same construction as in conventional high speed steel drills it is impossible to fully develop the performance of cemented carbide drills and they can be used only under moderate cutting conditions.
In drilling operations, the quality of the chip ejecting function or ability influences the cutting resistance. The greater the drilling depth, the greater the cutting resistance, thus making it necessary to improve the chip ejecting ability so as to prevent an increase in the cutting resistance. This is a matter of great importance particularly to cemented carbide drills.
The strength of drills is given by the toughness and rigidity of the material of which the bit is made and by its bending strength and rigidity and twisting rigidity which depend on the drill configuration.
FIG. 1 shows a conventional drill, looking axially at its cutting end, and the configurational elements will now be described with reference to FIG. 1. The portion indicated by a dashed line is a web portion 1, which is a solid portion where flutes 2 are not formed. That is, as is known in the art, the drill is spirally formed around the web portion 1 with flutes 2 serving as chip ejecting passages and land portions 3 which are thick-walled portions. The ratio c:d of the diameter (c d) of the web portion 1 to the drill diameter (shown by is referred to as the web thickness ratio c:d expressed in %, influences the drill strength and so does the ratio b:a of the circumferential length b of the flutes 2 to the circumferential length a of the land portions 3, referred to as the flute width ratio expressed by b:a. These two ratios are factors which influence the drill strength. FIG. 2 is a graph showing the twisting rigidity or strength as a function of the web thickness ratio with two different flute width ratios as parameters.
However, simply increasing the web thickness ratio and decreasing the flute width ratio increases the cutting resistance and makes the chip ejection difficult. Thus, the web thickness ratio and flute width ratio have their respective limits; generally, the web thickness ratio is set in the range 15% to 23% and the flute width ratio is in the range of 1:1 to 1.3:1.
The strength required of drills should be high enough to withstand the cutting resistance acting on the drill and hence the same effect of increasing the strength can be attained by decreasing the cutting resistance.
Referring to FIG. 1 showing the end view illustrating a conventional drill bit configuration, it is seen that the radial rake angle .theta..sub.r of the cutting lips 4 shows negative values at any position. Generally, the greater the rake angle, the lower the cutting resistance, which means that with conventional drills it is impossible to further decrease the cutting resistance.
Further, the fact that the radial rake angle .theta..sub.r of the cutting lips 4 is negative means that the cutting lips 4 extend increasingly backward with respect to the direction of rotation of the drill as they extend radially outward. Therefore, the relative distance between each cutting lip 4 and the flute wall 5 of the land portion 3 opposed thereto across the flute 2 increases. Where the relative distance is increased, as shown in FIG. 3, a chip 6 being cut by the cutting lip 4 sometimes fails to come in complete contact with the flute wall 5 and instead it extends to as far as the hole wall, which is the finished surface, thus increasing the cutting resistance or damaging the hole wall. Particularly in the case of deep hole drilling, the chip 6 itself chokes up the flute 2, making the chip ejection more difficult and greatly increasing the cutting resistance.
One of the important elements that govern such movement of the chip 6 is the relation between the shape or curve of the cutting lips 4 and the shape or curve of the flute walls 5. Thus, for smooth ejection of the chip 6 along the flute 2, there is an appropriate configurational relationship between the cutting lip 4 and the flute wall 5. One method of expressing the configurational relationship by some index is to represent the positional relation of the flute wall 5 relative to the cutting lip 4 or of the cutting lip 4 relative to the flute wall 5 by the relative distance therebetween. In this invention, this relative distance is defined to provide a measure for the quality of the chip ejection.
Further, in drills, the cutting lips 4 and the edges 7 of the land portions 3 are given a sharp form, and where a drill is made of cemented carbide which is relatively brittle, damage or breakage takes place most frequently in such portion, a fact which is attributable to a kind of configurational element.